Explanation: In the Siege of Alesia during the Gallic Wars of Julius Caesar (of conqueror and dictator fame), Caesar, with a force of about 50,000 Roman and allied troops encircled a slightly largely force under Vercingetorix in the fortified city of Alesia. Caesar, in order to ensure none of the Gauls could escape to regroup, built a wall around the fortified town. Vercingetorix was a charismatic and skilled Gallic warlord who sought to expel the Roman intruders from Gaul, after several years of the Romans increasingly intervening in inter-tribal wars and making themselves increasingly 'at home' as overlords of Gallic polities.
Vercingetorix, however, had planned to be surrounded - he had sent out the call for a much larger force to gather and surround Caesar while he was surrounding Vercingetorix!
Caesar played the Uno Reverse card, and built a wall around the wall he was using to surround the walled city of Alesia. That's three (3) walls in total, for those counting, two by Caesar, one by the Gauls. So when the massive Gallic relief force arrived, they found out that there was no fair fight to be had - they had to siege out Caesar's own besieging force to rescue Vercingetorix! To make matters worse for them, Caesar had his men ransack the countryside for all available food, burning what they couldn't take - meaning the relief force couldn't linger for long without starving.
By Caesar's counting, he faced nearly ~350,000 Gallic warriors in total. It was likely significantly less than this, with the number exaggerated for both practical (hard to count an enemy hemming you in while you hem them in) and propaganda (big numbers = big victory) reasons. Modern figures range from ~80,000-200,000 enemy troops against Caesar.
Forced into several costly assaults on the Roman walls by their circumstances, the Gauls, even coordinating between the besieging and besieged forces to time their attacks, were driven back, and Vercingetorix eventually surrendered for lack of supplies (and lack of ability to break out of Caesar's siege), ending the Gallic Wars with one of Caesar's greatest victories.